The field of this invention relates to processes for improving phosphorus-vanadium oxide and phosphorus-vanadium-co-metal oxide catalysts suitable for the oxidation of butane, the C.sub.4 hydrocarbons such as butene and butadiene, and benzene to maleic anhydride by treating the catalyst in situ with water and phosphorus compounds in sufficient quantity to cause the reaction exotherm or "hot spot" to shift farther into the catalyst bed.
Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins. It is also a versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs.
In general, catalysts proposed for the oxidation of butane, other C.sub.4 hydrocarbons, and benzene to maleic anhydride have been based upon vanadium and phosphorus. In U.S. Pat. No. 3,293,268, it is disclosed that the oxidation of butane to maleic anhydride can be performed in the presence of a phosphorus-vanadium-oxygen-containing complex catalyst. Though this catayst is capable of oxidizing butane, it does not give sufficiently high yields of maleic anhydride. Yields of maleic anhydride of only 30 to 50 weight percent are reported. Various activators, stabilizers, and promoters have been disclosed in the prior art to improve the yields of maleic anhydride. References include U.S. Pat. Nos. 3,867,411; 3,832,359; 3,888,886; 4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338; 4,152,339; 4,081,460; 4,043,943; 4,154,703; and British Application No. 2,019,839A. While the aforementioned prior art tends to bring about some improvement in the performance of the phosphorus-vanadium catalyst, there remains much room for improvement, particularly from the standpoint of high conversion, yield, and catalyst life. U.S. Pat. No. 4,081,460 relates to molybdenum-vanadium catalysts which are regenerated by the addition of phosphorus. Other references of interest include U.S. Pat. Nos. 4,002,174; 4,094,816; 4,089,807; 3,296,282; 3,474,041; and British Patent 1,464,198. While some of these references relate to catalyst regeneration, they do not relate to improving the performance of the catalyst by causing the reaction exotherm to shift farther into the catalyst bed using phosphorus compounds and water.
Our catalyst is suitably prepared in organic solvents by slurrying vanadium compounds and metal oxides, such as molybdenum oxide, zinc oxide, uranium oxide, tungsten oxide, tin oxide, and bismuth oxide, in organic solvents, preferably organic ether or alcohol solvents.
The object of the present invention is to improve the performance and extend the useful life of a phosphorus-vanadium oxide and a phosphorus-vanadium-co-metal oxide catalyst by adding phosphorus compounds in situ to the catalyst, thus causing the reaction exotherm "hot spot" to shift farther into the catalyst bed. A further object is to provide a process for the manufacture of maleic anhydride in the presence of the catalyst whose exotherm has shifted farther into the catalyst bed.
We have now discovered a method for increasing the selectivity of phosphorus-vanadium oxide and phosphorus-vanadium-co-metal oxide catalysts. According to our process, the vapor-phase oxidation of butane to maleic anhydride is conducted in a fixed-bed reactor by contacting the n-butane with oxygen and nitrogen in the presence of a vanadium-phosphorus-oxygen catalyst or a vanadium-phosphorus-oxygen catalyst promoted by a co-metal. The catalyst is treated in situ with water and phosphorus compounds in sufficient quantity to cause the reaction exotherm or "hot spot" to shift farther into the catalyst bed. On a given catalyst that has been extensively used, the yield of maleic anhydride is increased when the "hot spot" is moved farther into the catalyst bed where the catalyst is more selective. In a continuous or intermittent addition of phosphorus compounds and water to a catalyst whose yield has declined, the yield is increased by about 5 weight percent to about 15 weight percent without moving the "hot spot." When the "hot spot" is moved according to the process of the present invention, the yield is increased further by about 5 weight percent to about 20 weight percent. Optimally, with the exotherm at the new location, the in situ phosphorus addition is reduced or discontinued or the water addition is increased and a further improvement in the product yield is then obtained. This further increase in yield is due to an improved temperature profile in the catalyst bed over the addition of the phosphorus compound continuously and maintaining the "hot spot" in the same relative position. The improved temperature profile results when the partially deactivated catalyst in the original "hot spot" location reactivates. Usually, the yield improvement from this reactivation of the original "hot spot" catalyst is about 5 weight percent to about 20 weight percent.
The catalyst to be improved can be prepared in various ways including the one disclosed in U.S. Pat. No. 3,862,146, issued Jan. 21, 1975, having Edward M. Boghosian as its inventor. Alternatively, the catalyst to be improved can suitably be prepared from an alcohol solution which has been reacted with phosphorus pentoxide and has been saturated with an inorganic acid, such as hydrogen chloride. Other ways to prepare the catalyst are disclosed in U.S. Pat. No. 4,328,126 wherein the catalyst is made from an organic solvent system. Precipitation of the phosphorus-vanadium or phosphorus-vanadium-co-metal mixed oxide can suitably be effected by azeotropic distillation of the organic solvent and the water of reaction and the subsequent evaporation of the organic solvent. The atomic ratio of vanadium to phosphorus can suitably be in the range of about 0.5:1 to about 1.25:1, preferably in the range of about 0.6:1 to about 1:1. The total atomic ratio of the co-metal to vanadium advantageously is in the range of about 0.005:1 to about 0.25:1. The atomic ratio of phosphorus to vanadium is suitably in the range of about 2:1 to about 0.8:1, preferably about 1:1 to about 1.7:1. According to our process, the average valence of vanadium is in the range of about 3.8 to 4.2.
Addition of the phosphorus compound and water to the catalyst can suitably be conducted by adding water and the phosphorus compound, such as an alkyl ester of phosphoric acid, in a continuous or intermittent manner directly to the gaseous feed stream until the "hot spot" has been moved, and the addition is then terminated or reduced or the water addition is increased. The "hot spot" temperature is monitored by a thermowell and this records its movement. This method is particularly suitable in continuous processes which utilize multi-tubular upflow reactors.
The continuous improvement is applicable to phosphorus-vanadium oxide catalysts and to phosphorus-vanadium oxide catalysts promoted by metals. Suitable metals include molybdenum, zinc, tungsten, tin, bismuth, niobium, antimony, titanium, chromium, and uranium. The co-metal, such as molybdenum, zinc, tungsten, uranium, chromium, or tin may be added as a compound together with vanadium or separately introduced into the solution. Suitable co-metal compounds comprise their oxides and most soluble salts, such as the nitrates, chlorides, phosphates, sulfates, bromides, acetates, and others. Suitable molybdenum compounds comprise molybdenum oxide and most soluble molybdenum salts. If it is desired to improve physical properties of the catalysts, they may be treated with the suspension of an inert support, for example, alumina, titania, silicon carbide, kieselguhr, pumice, or silica. The catalyst may be reinforced with such materials at any stage in its preparation.
The vanadium compound can be vanadium pentoxide, vanadium tetrachloride, vanadium trichloride, vanadium oxydichloride, vanadium oxytrichloride, vanadium tetraoxide, vanadium oxalate, and most soluble vanadium complexes. Suitable vanadium compounds include: vanadium oxides, such as vanadium pentoxide, vanadium trioxide, and the like; vanadium oxyhalides, such as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide, and the like; vanadium-containing acids, such as meta-vanadic acid, pyrovanadic acid, and the like; and vanadium salts, such as ammonium meta-vanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate, and the like. However, vanadium pentoxide is preferred.
In our process, various phosphorus compounds may be utilized, but alkyl esters of phosphates are preferred. The catalyst exotherm can be shifted down the catalyst bed by adding about 0.005 to about 5 grams of phosphorus compounds per kilogram of catalyst per day and adding water to the feed gas stream giving a water concentration in this stream of about 1,000 to about 40,000 parts per million by weight based on the total weight of the feed gas stream.
The catalyst "hot spot" is moved farther into the catalyst bed, and its performance is improved by the addition of water and organophosphorus compounds, such as alkylphosphates, phosphites and phosphines. Representative phosphorus compounds have the following structure: ##STR1## wherein R is phenyl or an alkyl radical of one to 6 carbon atoms and X is H or R. Suitable compounds are: the primary, RPH.sub.2, secondary, R.sub.2 PH, and tertiary, R.sub.3 P, phosphines, such as ethyl phosphine; the tertiary phosphine oxides, R.sub.3 PO, such as tripropyl phosphine oxide; the primary, RP(O)(OX).sub.2, and secondary, R.sub.2 P(O)OX, phosphonic acids, such as benzene phosphonic acid; the esters of the phosphonic acids, such as diethyl methanephosphonate; the phosphonous acids, RPO.sub.2 X.sub.2, such as benzenephosphonous acid and the esters thereof, such as the monoethyl ester; the phosphinous acids, R.sub.2 POX, such as diethyl phosphinous acid and the esters thereof, such as the onoethyl ester; the primary, ROP(OX).sub.2, secondary, (RO).sub.2 POX, and tertiary, (RO).sub.3 P, phosphites, such as diethyl phosphite, trimethyl phosphite, triethyl phosphite, triisopropyl phosphite, tripropyl phosphite and tributyl phosphite, and the pyrophosphites, such as tetraethyl pyrophosphite. The preferred phosphorus compound is an ester of orthophosphoric acid having the formula (RO).sub.3 P.dbd.O wherein R is hydrogen or a C.sub.1 -C.sub.4 alkyl, at least one R being a C.sub. 1 -C.sub.4 alkyl. The preferred phosphate compounds are triethylphosphate or trimethylphosphate.
The "hot spot" management process for the improvement of the catalyst and the extension of the catalyst life may be initiated at any time. However, it is usually implemented when a decline in the selectivity of the catalyst is noted resulting in a decline in the yield from the process. The phosphorus compound and water can be added to the catalyst in a number of different ways. Usually, the catalyst will first be used for the oxidation of butane to maleic anhydride for a period of time until the yield of maleic anhydride diminishes. The phosphorus compound and water may then be added to increase the selectivity of the catalyst. This addition may be accomplished either with or without the continued flow of hydrocarbon and/or oxygen.
A preferred method for the improvement of the catalyst is the continuous or intermittent addition of a volatile organophosphorus compound and water to the gaseous stream of hydrocarbons and oxygen-containing gases entering the reactor. By such a technique, the activity of the catalyst is maintained through continuous improvement or stabilization. An advantage of this procedure is.that the production of maleic anhydride does not have to be interrupted.
Still another method for the addition of the phosphorus compound to the catalyst is the addition of the phosphorus compound in liquid phase by pouring the phosphorus compound over the catalyst to be improved. This techniqe may suitably be performed at about room temperature, if desired.
Thus, the phosphorus compound and water may be added to the catalyst by a variety of methods, such as adding the phosphorus compound as a liquid or gas. Other techniques, such as the use of an aerosol to convey the phosphorus compound, are also satisfactory. Suspensions or colloidal solutions of the phosphorus compounds may be employed. Solvents for the phosphorus compound, such as water, may be included. The phosphorus compound and water can be added with the hydrocarbon feedstock or via a diluent gas, such as nitrogen. The overall temperature range for the addition of the phosphorus compound and water suitably will be about from 0.degree. to 600.degree. C., depending upon the particular compound selected. However, the preferred temperature of the catalyst at the time of addition of the phosphorus compound and water will be about 300.degree. to about 500.degree. C., with still better results being obtained at a catalyst temperature of at least 350.degree. C. The upper limits of the temperature of the catalyst during the addition will suitably be about 450.degree. or 500.degree. C., or perhaps higher for short periods. The pressure during the addition may be atmospheric, subatmospheric or superatmospheric. The conditions of concentration, temperature and pressure should be adjusted to permit optimum contact of the phosphorus compound with the catalyst.
As pointed out above, the addition may be either intermittent or continuous. Of course, even if the phosphorus compound and water are added continuously, it is not necessary that they be added at a constant rate.
The phosphorus compound and water partially deactivate some portions of the catalyst. More specifically, the phosphorus partially deactivates the non-selective reaction exotherm and causes the reaction exotherm to migrate deeper into the catalyst bed in a beneficial manner. Note that the major portion of the catalyst bed is substantially unused; however, over a period of time that portion of the catalyst bed (fixed bed) wherein the reaction exotherm is located becomes less selective due to phosphorus loss with a corresponding drop in the yield of the anhydride product, and a corresponding increase in CO and CO.sub.2. Thus, according to the present invention, the phosphorus compound and water are added to partially deactivate the non-selective reaction exotherm and to allow a new exotherm to occur in the relatively unused portion of the bed. Hence, there will be an improvement obtained over the relatively non-selective reaction exotherm. This process may be repeated as long as there is sufficient catalyst bed into which the exotherm may migrate. It should be appreciated that too much of the phosphorus compound and water passed into the catalyst bed can deactivate the entire bed; thus, an appropriate method of obtaining the benefits of the present invention is to add only enough phosphorus compound to cause the reaction exotherm to move gradually, until the original exotherm is partially deactivated and the relocated exotherm is at an acceptable location.
It has also been discovered that by utilizing the process of this invention, the temperature profile of the catalyst bed is made more isothermal which leads to a further increase in the anhydride yield over only the moving of the exotherm. When the addition of the phosphorus compound is stopped or reduced or the water addition is increased after the exotherm has migrated deeper into the catalyst bed, the catalyst in the original exotherm location begins to reactivate. During this process, the ctalyst bed has a very broad "hot spot" having a much lower temperature than either the original exotherm or the new exotherm. The yield of the anhydride from this more isothermal catalyst bed is improved as the catalyst is more selective over the yield from the catalyst at the new exotherm location. As the original "hot spot" catalyst fully reactivates, the temperature profile returns to the original values.
It has further been discovered that by utilizing the process of this invention on a given catalyst, the temperature profile is made more isothermal during the addition of a phosphorus compound and water. The phosphorus compound and water shift the exotherm deeper into the catalyst bed but only partially deactivate the catalyst in the original "hot spot" location. Thus, the combined effects of a new exotherm location and a more isothermal catalyst bed lead to an increased anhydride yield during the application of the phosphorus compound and water.
This invention also comprises a process for oxidizing butane to maleic anhydride by contacting it in the presence of oxygen with the continuously improved catalyst and for improving the color and color stability of maleic anhydride produced by our novel process employing continuous or batch catalyst improvement. The catalyst improvement is conducted at a temperature of about 300.degree. C. to about 500.degree. C. The phosphorus compound or an aqueous medium comprising 0.01 to 100 weight percent of the phosphorus compound, more preferably about 25 to 75 weight percent of the phosphorus compound, is contacted with the feed gas stream flowing to the reactor. Alternatively, the phosphorus compound and water may be added directly to the butane feed prior to the mixing of the butane and air reactants. The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic mixtures of oxygen and diluent gases, such as nitrogen, may also be employed. Air enriched with oxyen may be used.
The gaseous feed stream to the oxidation reactors will normally contain air and about 0.2 to about 1.7 mole percent of n-butane. About 0.8 to 1.5 mole percent of n-butane is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of butane, less than about one percent, of course, will reduce the total yield obtained at equivalent flow rates and, thus, are not normally economically employed. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but the preferred range of operations is at the rate of about 100 to 4000 cc of feed per cc of catalyst per hour, and more preferably about 1000 to 2400 cc of feed per cc of catalyst per hour. Residence times of the gas stream will normally be less than about four seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury and 0.degree. C. A variety of reactors will be found to be useful and multiple-tube heat exchanger-type reactors are quite satisfactory. The tops of such reactors may vary in diameter from about one-quarter inch to about three inches, and the length may be varied from about three to about ten or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the surface of the reactors at a relatively constant temperature and some medium is needed to conduct heat from the reactors, such as lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate-sodium nitrite-potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metals surrounding the tube act as a temperature-regulating body. As will be recognized by one skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes, such as vycor, and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a preheat zone under an inert material, such as one-quarter inch Alundum pellets, inert ceramic balls, nickel balls or chips, and the like, present at about one-half to one-tenth the volume of the active catalyst present.
The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other media is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than 20.degree. to 50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and the butane concentration.
The reaction may be conducted at atmospheric, superatmospheric or subatmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to ensure a positive flow from the reaction. The pressure of the inert gases must be sufficiently high to overcome the pressure drop through the reactor.
Maleic anhydride may be recovered by a number of ways well-known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media, with specific operation and purification of the maleic anhydride. The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these examples are given for illustrative purposes only and will not be interpreted as limiting the invention in any way. In the examples, the terms "conversion", "selectivity" and "yield" are defined as follows: